Percent Versus Percentage

First let's get our terminology right. In some cases percent and percentage can be interchangeable (1), but the easiest way to choose the right word for the right situation is to use percent with a number and percentage without a number. For example,

[Percent with a number] Forty percent of the chocolate was missing

[Percentage without a number] What percentage of the chocolate was missing?

Percent Versus Per Cent

Also, in American English, when you write out the word percent, it's one word. It's more common to see the two-word version--per cent--in British English, but sources tell me the one-word version is becoming more common in Britain too (2, 3, 4). The evolution of the word is kind of interesting. It started out as the Latin phrase per centum, which means "by the hundred (1)," and over the years got shortened to the two-word English version, and is now quite established as a single English word (2).

Singular or Plural?

Now, earlier this afternoon Shaun Klein asked me on Twitter whether percentages are singular or plural, and as is so often the case, the answer is "It depends." If you're referring to a percentage of something, then that something determines whether you use a singular or plural verb. (In technical terms, that "something" is called the object of the preposition. The preposition is the word of.) Here's an example:

Forty percent of the chocolate is missing. (In that sentence the chocolate is singular so you use a singular verb.)

Forty percent of the chocolate chips are missing. (In that sentence the chocolate chips is plural so you use a plural verb.)

But what if there's no preposition or object of the preposition after the word percent? You've lost your clue! First, ask yourself if it's implied. If the implied phrase is singular use a singular verb, and if it's plural use a plural verb. Here's an example:

The chocolate chips were pillaged. [new sentenc Forty percent were missing. (In the second sentence, the plural phrase--the chocolate chips--is the implied object of the preposition, so you use the plural verb--were.)

In the next example, the implied object is singular so you use a singular verb:

The chocolate was pillaged. Forty percent was missing.

Finally, if you have no way to figure out whether the word percent is referring to something singular or plural, you can use whatever verb you like--singular or plural--it's that easy (5).

Web Bonus: Using the Word Percentage

It's a little more complicated with the word percentage. The same rules I just told you apply when you are talking about a percentage of something: singular something, singular verb; plural something, plural verb. But when you are talking about the percentage of something, then it is always singular (6).

A percentage of the chocolate chips were missing.

The percentage of chocolate chips missing was shocking.

Also, for percentage, the order of the sentence matters. If the percentage phrase comes later in the sentence, you need a singular verb (2).

A percentage of the chocolate chips were missing.

There is a large percentage of chocolate chips missing.

Words or Symbols

So now that you know how to use percents, let's talk about how to write percents in a sentence. Unlike what I told you last week for the general rules about writing numbers, for percentages it's better to use the numeral and not the word, even if it's a number less than 10. If you're writing about the 5% of chocolate chips that were damaged, use the numeral 5 and not the word five. The only time you would write out the word instead of using the number is if the number was at the beginning of the sentence. Then the rule about not starting a sentence with a numeral takes precedence and you write out the word (7, 8,9). [Note, some style guides disagree and say you can use the word or the numeral when writing out percents (10).]

[[AdMiddle]Next you have to decide whether to use percent the word or percent the symbol. This is kind of like the general rule about writing numbers. If you're writing a technical or scientific document, then most sources recommend that you use the symbol. If you are writing something where numbers are used less frequently, then it is more common to write the word percent. Ultimately, it's a style issue, so make a decision and stick with it. Just remember to use the numeral and not the word for the number.

Decimals

If you're talking about a percent that is less than one, make sure you put a zero before the decimal point. Write something like 0.2%, not just .2%. This is true for writing any numeral that is less than one whether it's a percent or not (10, 11). That little decimal point is too easy to miss without the zero in front of it.

Lies, D*** Lies, and Percentages

First, something can't decrease by more than 100%. Once 100% of something is gone, there isn't anything left. Never write that a price or anything else decreased by 150%. It's impossible*.

Second, when you are reading about medical, political, or financial news it is important to understand that big percentages can mean small overall increases or decreases. For example, an article that reports a 50% increase in the rate of a rare disease may be telling you that instead of 1 in 100,000 people getting floogety flork disease every year, now 1.5 people in 100,000 get the disease every year. A 50% increase sounds a lot scarier than the increase in raw numbers. Percentages aren't always misleading, but it's something to watch out for.

* Sigh. I should know better than to use absolute words like impossible. People have almost convinced me that when a value can become negative, it is possible for the value to decrease by more than 100%. What do you think? Join the discussion in the comments.